Shovel-nosed snakes and sandfish lizards both swim through granular materials like sand. Researchers at Georgia Tech used x-rays to observe their subsurface motions. Despite their different shapes, the long, slender snake and the shorter, wider lizard both move under the sand by projecting traveling waves along their bodies. The snake’s long, skinny body allows it to have more bends along its length, which increases its transport efficiency because it allows the snake to move mostly through the tunnel created by its head’s passage. In contrast, the sandfish’s motions fluidize the sand around it, enabling it to swim. Although the snake is faster, both animals have optimized their motions for fast, low-energy transit according to their body type. (Video credit: Georgia Tech; research credit: S. Sharpe et al.; via io9)
Tag: granular material
Raindrops on Sand
Here is a high-speed look at the impact of a raindrop on a sandy beach. In this case, a water droplet is falling on a bed of uniform glass beads, but the situation is effectively the same. Depending on the speed of the drop at impact, many types of craters are possible. The higher the impact velocity, the greater the momentum of the drop at impact and the more likely the drop is to tear apart when surface tension can no longer hold it together. Interestingly, there is remarkable similarity between the shape and behavior of these liquid drop impacts and those of a catastrophic asteroid impact. (Video credit: R. Zhao et al.)
Barchan Dunes
Crescent-shaped barchan dunes are common on both Earth (top image) and Mars (bottom image). They form in areas where the wind comes predominantly from one direction. As the wind blows, it deposits sand on the gently sloping windward face of the dune. The leeward face of the dune is steeper; its shape is set by the sand’s angle of repose–essentially the steepest angle the sand can maintain without an avalanche. Barchan dunes are very mobile, moving between one and a hundred meters per year. They have also been seen moving through one another or moving along in formation. (Image credits: Google Earth, NASA/JPL/University of Arizona)
Bouncing with Liquids and Grains
Bouncing a ball partially filled with a liquid can create chaotic results when the motion of the ball, fluid, and vibration plate couple. The behavior of a grain-filled ball is a bit different, though. Large grains will tend to bounce with the same frequency as the ball, even across a range of vibration conditions. A ball filled with smaller grains displays a variety of responses depending on the vibration conditions. Among these is a localized wave-like form called an oscillon which oscillates with a period different from but coupled to that of the vibration plate. All these different behaviors inside the bouncing sphere have noticeable effects on its outward motion, too. The chaotic activity of the fluid inside a bouncing ball makes it unstable, and, if not confined, it will bounce itself off the vibration platform. The grain-filled ball, on the other hand, remains bouncing on the platform even after being perturbed. This seems to be a result of the energy dissipation provided by the many inelastic collisions inside the ball as it bounces. (Video credit: F. Pacheco-Vazquez et al.)
Bead-Infused Droplet
A Leidenfrost droplet impregnated with hydrophilic beads hovers on a thin film of its own vapor. The Leidenfrost effect occurs when a liquid touches a solid surface much, much hotter than its boiling point. Instead of boiling entirely away, part of the liquid vaporizes and the remaining liquid survives for extended periods while the vapor layer insulates it from the hot surface. Hydrophilic beads inserted into Leidenfrost water droplets initially sink and are completely enveloped by the liquid. But, as the drop evaporates, the beads self-organize, forming a monolayer that coats the surface of the drop. The outer surface of the beads drys out, trapping the beads and causing the evaporation rate to slow because less liquid is exposed. (Photo credit: L. Maquet et al.; research paper – pdf)
Granular Jets
Object impacts in water and other fluids often create cavities that generate jets when they collapse. But impacts on granular materials can produce similar results, forming a cavity, a splash corona, and, under the right circumstances, a jet. This Sixty Symbols video explores the effect of grain size (and thus weight) on the formation of such a rebound jet. Ultimately, the jet behavior is driven by air. When the granular material is poured, air gets trapped between the grains. The impact compresses the grains, forcing the previously trapped air up and out through the cavity created by the impact. Interestingly, once the air pressure is low enough, jet creation is suppressed, not unlike splash suppression in liquids. (Video credit: Sixty Symbols/Univ. of Nottingham)
Granular Jet
Sometimes the similarity between fluid flow and granular flows is quite striking. This video shows a stream of sand falling down a tube and impacting a rod. (Note: the view is rotated 90 degrees counter-clockwise, so down points to the right.) As the sand strikes the rod, it’s deflected into a conical sheet, very much like a water bell. There are even ripple-like instabilities that form in the granular sheet, though they move differently than in a liquid due to the sand’s lack of surface tension. (Video credit: S. Nagel et al.)
Soil Liquefaction
Soil liquefaction is a rather unsettling process in which apparently solid ground begins moving in a fluid-like way after agitation. It occurs in loose sediments when the spaces between individual particles become nearly saturated with water. This can happen, for example, after heavy rains or in a place with inadequate drainage. Such cases are typically very localized, though, and require some significant agitation of the surface, like pressing with heavy machinery or jumping in a single spot. Soil liquefaction becomes a greater danger, however, in an earthquake. Even in a dry area, the earth’s shaking can force groundwater up into the surface sediment and vibrate the soil sufficiently to liquify it, causing whole buildings to sink or tip and wreaking havoc on manmade infrastructure. (Video credit: jokulhlaups)
Impacts on Sand
Granular materials like sand are sometimes very fluid-like in their behaviors. The high-speed video above shows a ball bearing being dropped into packed sand. Many features of the splash are fluid-like; the initial impact creates a spreading crownlike splash, followed by a strong upward jet that eventually collapses back into the medium. At the same time, many of the impact characteristics are decidedly non-fluidic. Sand has no surface tension, so both the crown and the jet readily break up into small particles. The granular jet is very narrow and energetic, reaching heights greater than the impacter’s drop height. Interestingly, the column begins collapsing on its lower end before the jet even reaches its highest peak. This may be due to the lower energy of the sand particles that were ejected later in the crater formation process. (Video credit: J. Verschuur, B. van Capelleveen, R. Lammerink and T. Nguyen)
Particle-Tracking in Granular Flows
One of the challenges of experimental fluid dynamics is gathering sufficient data in environments that can be fast-changing, visually dense, and sometimes harsh. Ideally, researchers want to gather as much data–velocities, temperatures, pressures–at as many points as possible and do so without disturbing the flow with a probe. No technique can provide everything, and thus new diagnostics are always under development. This video shows a new particle tracking method developed for fluidized granular flows where the high concentration of particles makes other techniques unsuitable. Such flows are often seen in industrial applications in chemical processing, pharmaceuticals, and powder transport. Interestingly, the technique can also be used in particle-seeded fluid flows like those normally studied with particle image velocimetry (PIV). (Video credit: F. Shaffer and B. Gopalan; submitted by @ASoutolglesias)
